01 - Sprinkle gelatin over 1/2 cup cold water in the mixer bowl and let it soften for 10 minutes.
02 - In a saucepan, combine remaining 1/2 cup water, granulated sugar, corn syrup, and salt. Heat over medium stirring until sugar dissolves. Increase heat and boil without stirring until mixture reaches 240°F.
03 - With mixer on low, slowly pour hot sugar syrup into the bloomed gelatin.
04 - Add vanilla bean seeds or vanilla extract. Increase speed to high and beat until mixture is thick, glossy, and has tripled in volume, about 10 to 12 minutes.
05 - Lightly oil a 9x13-inch pan and dust with confectioners sugar. Spread marshmallow mixture evenly, dust top with confectioners sugar, and let set uncovered at room temperature for 4 hours.
06 - Use a lightly oiled knife to cut marshmallows into large squares. Dust sides with confectioners sugar to prevent sticking.
07 - Whisk milk, cocoa powder, sugar, and salt in a saucepan. Heat over medium until steaming and smooth, do not boil. Remove from heat and stir in vanilla extract.
08 - Pour hot chocolate into mugs, top each with a marshmallow floater, add whipped cream if desired, and serve immediately.